VIRGIN Mobile customers will be moved across to O2 plans from March, the comms giant has revealed.
The operator will start informing the first batch of affected users about the changes from next month.
Virgin Media O2 said the shift will mean millions can benefit from "extra value from their services".
This includes things like unlimited texts, voice calls and O2's generous roaming perk - as the last of the big four to offer free roaming when travelling in EU countries.
The firm has assured customers that no-one will see the cost of their plan rise as a result.
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It comes a year and a half after the two companies became one in a £31billion mega-merger.
The "vast majority" of customers won't have to do anything like change their SIM - it'll all happen over the air.
Virgin's huge migration plan will go on until the end of the year.
It's not clear what will happen to the Virgin Mobile name after 2023 - the firm said it has a "variety of options under consideration" for the brand.
Virgin Mobile doesn't actually run the network managing everything behind-the-scenes - it's always "piggybacked".
Until the end of last year it ran off of Vodafone, before moving to O2 - so customers shouldn't notice any difference to the quality of their service.
"This is a major milestone moment for Virgin Media O2 as our Virgin Mobile customers start moving over to O2 plans, receiving added value and benefits on top," said Gareth Turpin, Chief Commercial Officer at Virgin Media O2.
"Our teams will guide customers through every step of the migration, and we’re laser focused on making sure this all occurs in the most hassle-free way possible.
"With all of our mobile brands now powered by the award-winning O2 network, we are making fantastic progress in our integration plans while continuing to deliver a range of knockout mobile services that cater for all needs."
Another perk Virgin Mobile customers will be able to take advantage of is the O2 Priority app.
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O2 Priority provides freebies like drinks from Greggs and cinema tickets for Odeon.
It also lets people get access to show tickets in advance, as well as the ability to jump the queue at O2 venues.
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